1. Field of the Invention
This invention relates to a rotating detecting apparatus suitable to detect a rotational status of a rotator such as a capstan motor for use in a video tape recorder, etc.
2. Description of Related Art
FIG. 1 is a plan view of a conventional capstan motor for a VTR (video tape recorder), which partially contains a broken-out section, and FIG. 2 is a cross-sectional view of the capstan motor as shown in FIG. 1. In the capstan motor as shown in FIGS. 1 and 2, a main magnet 2 is disposed inside of a rotor 1, and a coil 4 is disposed at a stator 3 side so as to be faced to the main magnet 2. With this construction, the main magnet 2 and the rotor 1 are rotated in accordance with polarization shift of the coil 4.
In this capstan motor thus constructed, an FG (frequency generator) magnet 5 on which N-poles and S-poles are alternately magnetized at a predetermined interval of .lambda./2 is further provided on the peripheral surface of the rotor 1, and a sensor 11 having a magnetoresistive (MR) element 11a for sensing a magnetic field generated by the N-poles and the S-poles of the FG magnet 5 is also disposed at the stator 3 side.
As shown in FIG. 3(B), the magnetoresistive (MR) element 11a on the sensor 11 includes four magnetoresistive units A', B', C' and D' each comprising a rectangular thin-film magnetoresistive element, which are disposed on a plane at an approximately quarter interval of a magnetization interval .lambda. of the same poles (for example, N-poles) of the FC magnet 5. The equivalent circuit to the magnetoresistive element 11a is a bridge circuit as shown in FIG. 3(C). That is, the magnetoresistive element 11a is constructed by the loop-connection of the magnetoresistive units A', B', C' and D' in a clockwise direction (in clockwise order). The connection point of the magnetoresistive units B' and C' is connected to a terminal for outputting an FG signal P' as described later, and the connection point between the magnetoresistive units A' and D' is connected to a terminal For outputting an FG signal P as described later. Further, the connection point between the magnetoresistive units A' and B' is connected to a power source Vcc, and the connection point between the magnetoresistive units C' and D' is connected to a ground GND (grounded).
In the magnetoresistive element 11a on the sensor 11 thus constructed, the magnetoresistive units such as the magnetoresistive units A' and C' as shown in FIG, 3(A) to which magnetic field in an Y-direction is applied From the FG magnet 5 are decreased in resistance value, and the magnetoresistive units such as the magnetoresistive units B' and D' to which magnetic field in an X-direction is applied From the FG magnet 5 are invariable in resistance value.
Therefore, as the FG magnet 5 is rotated together with the rotor 1, the potential at the connection point between the magnetoresistive units B' and C' (FG signal P') or the potential at the connection point between the magnetoresistive units A' and D' (FG signal P) in the equivalent circuit as shown in FIG. 3(C) are increased or decreased. Therefore, the sensor 11 can detect the rotational status of the rotor 1 (FG magnet 5) on the basis of the FG signal P' or FG signal P.
The potential at the connection point between the magnetoresistive units B' and C' (FG signal P') and the potential at the connection point between the magnetoresistive units A' and D' (FG signal P) in the equivalent circuit as shown in FIG. 3(C) are varied (increased or decreased) oppositely to each other (that is, when one is increased, the other is decreased), and thus these signals P and P' are signals whose phases are different From each other by 180.degree. C. (that is signals having opposite-phase signals). Accordingly, the conventional device has generally adopted a method that S/N of the FG signal is beforehand improved by, obtaining a difference between the FG signal P' and the FG signal P, and then the rotational status of the rotor 1 (FG magnet 5) is detected.
Therefore, in this device, the difference between the FG signal P' and the FG signal P is beforehand obtained, then a zero-cross point of the difference signal thus obtained is counted, and then a rotating speed of the rotor 1 (main magnet 2) is detected. In order to design the device in a compact size, the outer diameter of the FG magnet 5 is required to be small. However, the magnetization pitch .lambda. between the same poles can not be set to be small. Therefore, when the outer diameter of the FG magnet 5 is set to a small value without varying the magnetization pitch of the Fg magnet 5, the rate of the magnetization pitch .lambda. the outer diameter of the FG magnet is increased, and thus a period of the FG signal P' and the FG signal P is increased (frequency is lowered). Since the phase difference between the FG signal P' and the FG signal P is 180.degree., the number of zero-cross points of the difference signal between the FG signals P' and P is reduced.
In a case where the number of zero-cross points of the difference signal between the FG signals P' and P per one-rotation of the FG magnet 5 is reduced as described above, the zero-cross point number is further reduced when the rotor 1 is rotated at a lower speed for example. Therefore, there occurs a problem that the detection accuracy For the rotating speed of the rotor 1 (main magnet 2) is deteriorated. In addition, in a case where the rotating speed off the rotor 1 (main magnet 2) is controlled with the zero-cross point number as described above, there occurs another problem that the control of the rotating speed is difficult at a low-speed area.